1. Technical Field
The present invention relates to Global Positioning Systems in general, and particularly to a method for reducing positioning time of a Global Positioning System receiver.
2. Description of Related Art
Portable electronic devices, such as laptop personal computers (laptop PCs), tablet computers, smartphones, etc., are commonly equipped with a Global Positioning System (GPS) receiver. The GPS receiver analyzes navigation data carrying radio waves of an L1 wave (1,575.42 MHz) and an L2 wave (1,227.60 MHz) transmitted from GPS satellites for calculating the latitude and longitude thereof.
The navigation data are composed of 25 frames including ephemeris data indicative of precise orbit information on each individual satellite, almanac data indicative of coarse orbit information on all traveling GPS satellites, correction data for a satellite 3D clock, ionosphere correction data, and the like. The almanac data have a lot of information and hence is divided and contained in the 25 frames. Since the number of bits of each frame is 1,500 bits and the data rate of a navigation message is 50 bits per second (bps), the GPS receiver requires 30 seconds to receive one frame, requiring 12.5 minutes to receive the entire navigation data.
When the GPS receiver is powered on, the GPS receiver performs positioning through processes called acquisition and tracking. During the acquisition process, four or more GPS satellites capable of being used for positioning are searched for to acquire each set of ephemeris data. The more the acquired GPS satellites, the more the positioning accuracy is improved. During the tracking process, the distance to each of the to acquired GPS satellites is calculated from the navigation data received from the GPS satellites to perform positioning by triangulation based on the position of each GPS satellite.
The life of ephemeris data is approximately 2 hours and the life of almanac data is approximately 24 hours. The ephemeris data and the almanac data held by the GPS satellites are updated by a ground control station periodically. The GPS receiver internally stores the received navigation data for use in the next acquisition process so that positioning can be performed in a short time. However, if the operation stopping time becomes longer, the life of the above-mentioned data may expire from being used for subsequent positioning.
Starting positioning in a state that the GPS receiver already has effective ephemeris data and almanac data is called a hot start. Starting positioning in a state that the GPS receiver has only effective almanac data is called a warm start. Starting positioning in a state that the GPS receiver has neither effective ephemeris data nor almanac data is called a cold start.
A cold start requires a few minutes to receive almanac data in the acquisition process even when various algorithms are introduced. During the acquisition process, when a bit error occurs and hence the navigation data are fragmented, the navigation data need to be received again. A warm start requires 30 to 40 seconds to receive each frame of ephemeris data from the four or more GPS satellites in parallel during the acquisition process, and if an error occurs along the way, the other frames transmitted by the GPS satellites will need to be fully received again. There is a technique called an assist-type GPS, in which a radio base station or a network server periodically supplies the GPS receiver with ephemeris data and almanac data acquired from GPS satellites.
There exists a prior art technique for accurately measuring only the noise level of an ambient environment to determine a threshold value level in order to improve the acquisition accuracy of a received GPS signal. A GPS receiver is started a predetermined time after the start of on-board equipment. Since the GPS receiver measures noise only from the ambient environment without noise transiently radiated during start-up of the on-board equipment, and compares the set threshold value with an output signal from a correlator, a situation that the threshold value level becomes unstable such as to rise or fall unusually to disable the calculation of an accurate position is prevented. These acquisition sensitivity level is required to be higher than the tracking sensitivity level.
The GPS radio waves are a faint signal and the signal level is lower than a noise level in an environment where the GPS receiver exists. In one example, although a signal level in the position of a GPS antenna is about −120 dBm to −165 dBm, the noise level in the GPS receiver reaches −100 dBm. In the acquisition process during the cold start and the warm start, when a bit error occurs while a frame is being received, since the frame needs to be received again, it is necessary to keep the noise level of an ambient environment to the GPS receiver as low as possible in order to complete the acquisition process in the shortest time. On the other hand, since positioning can be performed in the tracking process even when time information is acquired in fragments, there is no need to fully receive the entire frame, resulting in higher noise tolerance than that in the acquisition process.
As an example, more than ten to twenty times the signal level is required in the acquisition process compared with the tracking process. A certain type of GPS receiver requires a signal level of about −140 dBm in the acquisition process and a signal level of about −160 dBm in the tracking process. When the GPS receiver is operated in an environment with a high noise level, positioning may be disabled because the acquisition process cannot be completed. Especially, noise from various devices can easily enter a GPS receiver stored in portable electronic devices.
The assist-type GPS cannot be used in an environment where no wireless network can be used, and the cost of communication for acquiring navigation data is also required. Further, even if ephemeris data and almanac data are received from the network, since there is a need to receive navigation, data from GPS satellites in the end, it is better even for the assist-type GPS to operate in an environment with a low noise level so that the acquisition process can be completed in a short time.
Consequently, it would be preferable to provide an improved method for reducing the positioning time of a GPS receiver within a noisy environment.